1. Field of the Invention
The present invention relates to telecommunication systems. In particular, the invention relates to a new and improved method of implementing different signal processing facilities in a digital transmission system environment.
2. Description of the Prior Art
At present, speech is being transferred in telecommunication networks generally in a digital form. The telecommunication network, especially a telephone network, consists of the terminal devices of the subscriber, telephones and digital transmission systems. One example of a digital transmission system is the DX 200 transmission system manufactured by the assignee. Its main parts and the principal structure are a subscriber interface, junction line interface, switching field, call control and operation control.
The subscriber interface is implemented by a subscriber stage which is responsible for connecting the subscriber lines to the transmission system. The junction line interface is implemented by a central terminal which comprises an interface to the transmission path and to the switching field. The switching to the switching field may be either a direct interface or an interface implemented indirectly via some other unit, such as an echo-cancelling unit.
In the digital transfer and transmission technique, a pair of wires has generally been used for the signal transmission that enables one to transfer the signal even longer distances without causing any significant crosstalk disturbance to its environment and without the signal itself being disturbed by an external energy radiation. The channel used for the through connection of a separate communication, for example a call, is transferred as multiplexed in the digital signals having a bigger transfer rate. Multiplexed signals may be transferred from one place to another using varying methods. The transmission systems handle in most of the cases bit streams with the transfer rate of 2.048 M bit/s (in the USA, Canada and Japan 1.544 Mbit/s).
When in the following section one speaks about the signal and echo of a call connection, the signal is primarily used to mean a PCM coded speech signal with the transfer rate of 64 kbit/s that is defined in the recommendation G.711. of ITU-T. In the systems with the transfer rate of 2.048 Mbit/s, which are hereinafter referred to as E1 lead or E1 interface, a PCM coding in accordance with the A-law is used, and in the systems with the transfer rate of 1.544 Mbit/s, which are hereinafter referred to as DS1 lead or DS1 interface, a PCM coding in accordance with the μ-law is used.
At present, echo erasing is needed primarily in two cases: in international telecommunication connections and mobile communication systems. The main technical reason for the fact that an echo eraser is needed is the 2/4 lead transformation, which is done in the line interface between the subscriber line and the local transmission system. Originally, the concept of “two-wire line” in the telephone technique means that both of the transmission directions of the signal proceed in the same twin wire. The “four-wire line” means that the directions have been so divided that the first direction uses the first twin wire and the second direction the second twin wire.
Since it is not reasonable or even possible to use a completely ideal impedance adaptation in the 2/4 lead transformation, for example for cost reasons, the signal leaks or “echoes” through the 2/4 lead transformation circuit to the transmission path of the other direction. In FIG. 1 this is illustrated so that in the transformation circuit 2/4WB of the B-subscriber, the leakage has been marked with a curved arrow, which represents a returning signal. Because of this returning signal, the A-subscriber hears his/her own speech. This does not do any harm, if the distance between the A-subscriber-2/4WB transformation circuit is relatively short so that the delay of the speech signal is below 25 ms. In practice, the distance and the delay caused by it exceed the allowed distance and delay. This happens often with the international connections and in mobile communication systems.
In the diagrammatic representation illustrated in FIG. 2, the tasks of the echo eraser EC include erasing the echo, that is the signal which in FIG. 2 is described by a curved arrow and letter e. When the echo e has been erased, the A-subscriber does not hear it any longer. The erasing of echo is possible because the leakage caused by the 2/4 lead transformation can be measured at the beginning of the call connection and during it, in which case the call eraser EC may model the leakage and based on it erase the echo signal from the signal to be transmitted.
Since the modern transmission systems are digital, often the whole distance between the echo eraser and 2/4 lead transformation is digital. The digital signal processing technique offers excellent possibilities for the advantageous implementing of the echo eraser. FIG. 2 illustrates also the switching of the signaling path through the transmission system. Normally, if the echo eraser is not used, then Ss2=Ss1 and Sr2=Sr1. When the echo eraser is in use, then Ss2=Ss1 but Sr2=Sr1−e, in which letter e stands for echo.
FIGS. 3a–3c represent alternative ways of implementation, of how to attach the echo-cancelling unit to the existing transmission systems. In describing the ways of implementation, the redundancy of the echo erasing facility is also taken into account, which is necessary because, if the echo-cancelling unit handling the traffic of a STM-1 line is corrupted, then a remarkably big traffic capacity is lost; the capacity of a STM-1 line is approximately 2000 simultaneous call connections with the transfer rate of 64 kbit/s.
FIG. 3a represents an example in which an echo eraser is used in a pool connected to the switching field. The interface unit or the STM-1 interface unit, that is the central terminal ETS1, by which the junction lines are attached to the transmission system and which in this example has been secured by a doubling, is attached to the switching field GSW of the telephone exchange. For the echo erasing there are also the echo-cancelling units ECS1 attached to the switching field. Among the call connections to be connected to the STM-1 line, the channels of those connections that need the echo erasing are circulated via the group ECS1 POOL of the echo-cancelling unit using the switching field GSW. For the redundancy, the echo-cancelling pool may be equipped with a desired amount of redundant echo-cancelling units. The disadvantage of the solution is the extravagant use of the access links of the switching field GSW.
FIG. 3b represents an example in which the echo erasers are located in a series between the switching field and the interface unit. In this solution only a third part of the access links of the switching field are used as compared to the case as presented by FIG. 3a, if we are examining a case in which all the STM-1 lines need echo erasing. The disadvantage with this solution is the cumbersome redundancy of the echo eraser; it has to be done for each ETS1 unit separately. Consequently, if the echo-cancelling unit has been integrated for one ETS1 unit, then one has to use an expensive 1+1 redundancy.
In the implementation as shown in FIG. 3c, the interface unit ETS1 is directly connected to the switching field unit GSW. The echo-cancelling units ECS1 are located under the control of the interface units ETS1 so that the connections needing the echo erasing are connected via the echo eraser before connecting them to the switching field. The disadvantage of the solution is the implementation of redundancy, which again has to be done for each ETS1 unit separately.
The disadvantage with all the solutions of echo erasing presented above is either the extravagant use of the access links of the switching field GSW or the non-optimal and expensive implementation of echo erasing.